1. Field of the Invention
The present invention relates generally to cardiac pacing using an implantable cardiac stimulator, and more particularly to verification of capture of the heart following application of an electrical stimulating pulse by the cardiac stimulator.
2. Background Information
A cardiac stimulator, or pacemaker, "captures" the heart by delivering an electrical pulse to the myocardium of a selected chamber during an interval in the cardiac cycle in which the cardiac tissue is excitable. The electrical pulse causes depolarization of cardiac cells and a consequent contraction of the chamber, provided that the energy of the pacing pulse as delivered to the myocardium exceeds a threshold value.
It is desirable to adjust the pacemaker so that the energy delivered by the electrical pulse to the myocardium is at the lowest level that will reliably capture the heart. Such a level assures therapeutic efficacy while maximizing the life of the pacemaker battery. Because the threshold for capture varies from one implantation to another, and can change over time, it is also desirable that the pulse energy delivered by the pacemaker to the myocardium be adjustable during and subsequent to implantation. Adjustment can be effected manually from time to time through use of an external programmer that communicates with the implanted pacemaker. It would be more desirable, however, to provide a pacemaker that adjusts the pulse energy itself automatically and dynamically in response to changes in the capture threshold.
Changes in capture threshold can be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy level should be increased. On the other hand, if capture occurs consistently at a particular stimulation level over a relatively large number of successive stimulation cycles, it is possible that the stimulation threshold has decreased and that pacing energy is being delivered at an energy level higher than necessary. This can be verified by lowering the stimulation energy level and monitoring for loss of capture at the new energy level.
For automatic and dynamic adjustment of the stimulation energy level to be successful, it is necessary for the implantable cardiac stimulator to be able to verify that capture has occurred. Capture verification is generally accomplished by detecting an electrical potential in the heart evoked by the stimulating pulse. If capture has not occurred, there will be no evoked potential to detect. It follows that each time a stimulating pulse is delivered to the heart, the heart can be monitored during an appropriate period of time thereafter to detect the presence of the evoked potential, and thereby verify capture. In practice, however, reliable detection of the evoked potential is not a simple matter, especially where it is desired to sense the evoked potential with the same electrode that delivers the stimulating pulse. This is because the evoked potential is small in amplitude relative to the residual polarization charge on the electrode resulting from the stimulation pulse. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds thereafter. Several techniques for alleviating the effects of the residual charge are disclosed in the prior art.
U.S. Pat. No. 4,858,610, issued Aug. 22, 1989, to Callaghan et al., teaches the use of charge dumping following delivery of the stimulating pulse to decrease lead polarization and also the use of separate pacing and sensing electrodes to eliminate the polarization problem on the sensing electrode. U.S. Pat. No. 4,686,988, issued Aug. 18, 1987, to Sholder, teaches the use of a separate sensing electrode connected to a detector for detecting P-waves in the presence of atrial stimulation pulses, wherein the P-wave detector has an input bandpass characteristic selected to pass frequencies that are associated with P-waves. U.S. Pat. No. 4,373,531 teaches the use of pre-and post-stimulation recharge pulses to neutralize the polarization on the lead. U.S. Pat. No. 4,537,201 teaches a linearization of the exponentially decaying sensed signal by applying the sensed signal through an anti-logarithmic amplifier in order to detect a remaining nonlinear component caused by the evoked potential. U.S. Pat. No. 4,674,509, issued Jun. 23, 1987, to DeCote, Jr. teaches the generation of paired pacing pulses spaced such that at most only one pulse of each pair can induce capture. The waveforms sensed through the pacing lead following the generation of each of the pair of pulses are electronically subtracted to yield a difference signal indicative of the evoked cardiac response.
Each of the prior art approaches to detecting a small-amplitude evoked potential in the presence of a large amplitude residual charge from a stimulating pulse has significant disadvantages. Those techniques that depend upon the use of a separate electrode located at some distance from the stimulating electrode so as to be isolated from the residual stimulating charge attempt to avoid the detection problem at the cost of requiring a separate sensing electrode. Those approaches that depend upon delivering an opposite-polarity charge to the electrode to neutralize the residual charge, and those approaches that depend upon delivering a pair of close-spaced pacing pulses are unnecessarily wasteful of battery power as well as being unduly complex. The approach that depends upon use of an anti-logarithmic amplifier to compensate for the generally exponential decay of the residual charge requires unnecessarily complex circuitry that is difficult to implement.
It would be desirable to provide a relatively simple and easily implemented capture verification circuit for use in an implantable cardiac stimulator that would permit detection of cardiac evoked potentials in the presence of a residual charge from a preceding stimulation pulse, and that permits use of the same electrode to sense the evoked response as was used to deliver the stimulation pulse. This and other desirable goals are met by the present invention.